It is well known that X-rays can cause appropriate substances to luminesce. Substances showing the phenomenon of luminescence under the influence of X-rays are called phosphors.
According to a classical radiographic method photographic silver halide emulsion film is combined with a sheetlike element, called intensifying screen that contains a particulate phosphor distributed in a binder layer to convert X-rays into fluorescent light whereto the film is much more sensitive than to the direct exposure with X-rays.
The phosphors used in said screens should have on X-ray exposure a high prompt emission but not exhibit appreciable long lived emission after the X-ray exposure is switched off.
According to another method of recording and reproducing an X-ray pattern disclosed e.g. in U.S, Pat. No. 3,859,527 and 4,258,264 a special type of phosphor is used, known as a photostimulable phosphor, which being incorporated in a panel is exposed to incident pattern-wise modulated X-rays and as a result thereof temporarily stores therein energy stemming from the X-ray radiation pattern. At some interval after the X-ray exposure, a beam of visible or infra-red light scans the panel to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which are processable to produce a visible image. For this purpose, a stimulable phosphor should store as much as possible of the incident X-ray energy and release as slowly as possible the stored energy until stimulated by the scanning beam. In other words the phosphor should have a high energy storage capacity with slow dark decay. In addition thereto the release of the stored energy on photo-stimulation should proceed efficiently, i.e. almost all the stored energy should be released within reasonable readout times in the form of stimulation light having a wavelength range sufficiently apart from the wavelength range of the stimulating light. In practice the stimulable phosphor should have high sensitivity to stimulating light in the wavelength range of 500 to 700 nm wherein a He--Ne laser (633 nm), ruby laser (694 nm), frequency doubled neodymium YAG laser (532 nm) or argon ion laser (514.5 nm) are emitting. From these commercial lasers the argon ion laser is the most powerful and offers in seqential exposure (scanning) the possibility to shorten the readout time.
The light emitted by stimulating the phosphor, called stimulation light, is situated preferably in the wavelength range of 300 to 500 nm where photomultiplier tubes have high quantum efficiency (ref. the periodical Radiology, September 1983, p. 834) and the stimulation light can be separated efficiently from the stimulating light by means of a proper cut-off filter.
As described in the above mentioned periodical the imaging plate containing a stimulable phosphor can be used repeatedly to store X-ray images simply by flooding it with light to erase the residual energy it contains.
Another important property for a practically useful phosphor is its chemical stability, more particularly its resistance to moisture, since hygroscopic phosphors suffer from loss in luminescence power when becoming wet.
As can be learned from U.S. Pat. No. 4,587,036 some europium-doped alkaline earth fluorohalides (described in EP No. 0 021 342) are sensitive to moisture whereby their fluorescence power is reduced.
In published European patent application No. 0 304 121 Eu.sup.2+ and/or Ce.sup.3+ doped (activated) halosilicates are described which are particularly resistant to moisture and have a high conversion efficiency of stored X-ray energy into visible stimulation light on stimulation with argon ion laser light of 514.5 nm.
In U.S. Pat. No. 5,034,607 Eu.sup.2+ or Ce.sup.3+ doped (activated) halogermanates and halo(silicate-germanate) phosphors optionally co-doped with Na.sup.+, K.sup.+ or Li.sup.+ are described which are particularly resistant to moisture and have a high conversion efficiency of stored X-ray energy into visible stimulation light on stimulation with light of wavelength range beyond 600 nm, so for He--Ne laser light of 633 nm.
As is generally known most phosphors contain in their empirical formula structure one or more host metals and a dopant or activator metal in admixture with one or more co-dopants. The host metal gives the phosphor X-ray stopping power and in conjunction with the dopant and co-dopants determines its energy storage capacity. The fluorescent light emission spectrum and the efficiency of the transformation of absorbed X-ray energy into emitted fluorescent light on stimulation are determined mainly by the dopant and the optionally present co-dopants.